Bars In Disk Galaxies Research Articles

A SAURON look at galaxy bulges

AbstractKinematic and population studies show that bulges are generally rotationally flattened systems similar to lowluminosity ellipticals. However, observations with state‐of‐the‐art integral field spectrographs, such as SAURON, indicate that the situation is much more complex, and allow us to investigate phenomena such as triaxiality, kinematic decoupling and population substructure, and to study their connection to current formation and evolution scenarios for bulges of early‐type galaxies. We present the examples of two S0 bulges from galaxies in our sample of nearby galaxies: one that shows all the properties expected from classical bulges (NGC5866), and another case that presents kinematic features appropriate for barred disk galaxies (NGC7332). (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Diffuse and Gravitationally Stable Molecular Gas in the Post-Starburst Galaxy NGC 5195

Abstract The Nobeyama Millimeter Array (NMA) has been used to make aperture synthesis CO($1-0$) observations of the post-starburst galaxy NGC 5195. CO($1-0$) and HCN($1-0$) observations of NGC 5195 using the Nobeyama 45 m telescope are also presented. High-resolution (${1\rlap {.}{}^{\mathrm {\prime \prime }}9} \times {1\rlap {.}{}^{\mathrm {\prime \prime }}8}$ or $86 \,\mathrm{pc} \times 81 \,\mathrm{pc}$ resolution at $D = 9.3 \,\mathrm{Mpc}$) NMA maps show a strong concentration of CO emission toward the central a few $\times 100 \,\mathrm{pc}$ region of NGC 5195, despite the fact that the current massive star formation is suppressed there. The face-on gas surface density, $\Sigma_\mathrm{gas}$, within the $r<2{}^{\prime\prime}$ or 90 pc region reaches $3.7 \times 10^3 M_\odot$ pc$^{-2}$ if a Galactic $N_\mathrm{H_2}/I_\mathrm{CO}$ conversion factor is applied. The extent of the central CO peak is about $5{}^{\prime\prime}$, or 230 pc, and is elongated along the E-W direction with two-armed spiral-like structures, which are typical for barred disk galaxies. The HCN-to-CO integrated intensity ratio on the brightness temperature scale, $R_\mathrm{HCN/CO}$, is about 0.02 within the central $r < 400$ pc region. This $R_\mathrm{HCN/CO}$ is smaller than those in starburst regions by a factor of $5 \hbox{--} 15$. These molecular-gas properties would explain why NGC 5195 is in a post-starburst phase; most of the dense molecular cores (i.e., the very sites of massive star formation) have been consumed away by a past starburst event, and therefore a burst of massive star formation can no longer last, although a large amount of low density gas still exists. We find a steep rise of the rotation velocity toward the center of NGC 5195. As a consequence, the critical gas surface density for a local gravitational instability of the gas disk becomes very high ($\Sigma_\mathrm{crit} \sim 6.9 \times 10^3 \,{{{M}_{\odot}}} \,\mathrm{pc}^{-2}$), suggesting that the molecular gas in the central region of NGC 5195 is gravitationally stable, in contrast to that of starburst galaxies. We propose that dense molecular gas can not be formed from remaining diffuse molecular gas because the molecular gas in the center of NGC 5195 is too stable to form dense cores via gravitational instabilities of diffuse molecular gas. The deduced very high threshold density seems to be due to a high mass concentration in NGC 5195. The known trends on the occurrence and luminosity of nuclear star formation in early-type galaxies can be understood naturally if the high threshold density is characteristic for early-type galaxies.

Nested Bars in Disk Galaxies: No Offset Dust Lanes in Secondary Nuclear Bars

Under certain conditions, subkiloparsec nuclear bars form inside large-scale stellar bars of disk galaxies. These secondary bars spend a fraction of their lifetime in a dynamically decoupled state, tumbling in the gravitational field of the outer bars. We analyze the flow pattern in such nested-bar systems under the conditions of negligible self-gravity and find that secondary bars differ fundamentally from their large-scale counterparts in gas flow pattern and other dynamical properties. In particular, the gas flow across the bar-bar interface in these systems can be more chaotic or more regular in nature and, contrary to predictions, has no difficulty in penetrating the secondary bars along the primary large-scale shocks. The outer parts of both short and long nuclear bars (with respect to their corotation) appear to be depopulated of gas, while deep inside them the flow exhibits low Mach numbers and follows oval-shaped orbits with little dissipation. Long nuclear bars remain gas-rich longer and for this relatively short period of time are largely of a rectangular shape, again with a small dissipation. We find that gas-dominated and star-dominated nuclear bars avoid the bar-bar interface, making both types of bars short relative to their corotation. Furthermore, our earlier work has shown that dynamically coupled secondary bars exhibit a similarly relaxed low-dissipation flow as well. Therefore, no large-scale shocks form in the nuclear bars, and consequently, no offset dust lanes are expected there. We find that offset dust lanes cannot be used in the search for secondary (nuclear) bars. Finally, we discuss the importance of gas self-gravity in the further evolution of these systems.

Double Bars in Disk Galaxies: Dynamical Decoupling of Non–Self‐gravitating Gaseous Bars

We find that nuclear rings in barred galaxies can be subject to a new type of non-self-gravitational dynamical instability. The instability leads to the formation of gaseous molecular bars with pattern speeds that are substantially slower than speeds of the primary stellar bars. This spectacular decoupling of nuclear bars from the underlying gravitational potential is triggered but is not driven by the gas viscosity. We find that low-viscosity systems can spend a substantial period of time in a fully decoupled state, with the nuclear bar slowly tumbling in the gravitational field of the primary bar. Higher viscosity systems form nuclear bars that librate about the primary bar. The shape of a nuclear bar, i.e., its eccentricity, correlates strongly with the angle between the bars. We also find that such decoupling, partial or full, most probably will be associated with bursts of star formation and with gas inflow across the inner (Lindblad) resonance zone toward smaller radii.

A Dust‐penetrated Classification Scheme for Bars as Inferred from Their Gravitational Force Fields

The division of galaxies into (SB) and (S) spirals is a fundamental aspect of the Hubble galaxy classification system. This tuning fork view was revised by de Vaucouleurs, whose classification volume recognized apparent (SA, SAB, SB) as a continuous property of galaxies called the family. However, the SA, SAB, and SB families are purely visual judgments that can have little bearing on the actual bar strength in a given galaxy. Until very recently, published bar judgments were based exclusively on blue light images, where internal extinction or star formation can either mask a bar completely or give the false impression of a bar in a nonbarred galaxy. Near-infrared camera arrays, which principally trace the old stellar population in both normal and barred galaxies, now facilitate a quantification of bar strength in terms of their gravitational potentials and force fields. In this paper, we show that the maximum value, Qb, of the ratio of the tangential force to the mean axisymmetric radial force in a barred disk galaxy is a quantitative measure of the strength of a bar. Qb does not measure bar ellipticity or bar shape but rather depends on the actual forcing due to the bar embedded in its disk. We show that a wide range of true bar strengths characterizes the category SB, while the de Vaucouleurs category SAB corresponds to a narrower range of bar strengths. We present Qb values for 36 galaxies, and we incorporate our bar classes into a dust-penetrated classification system for spiral galaxies.

The effect of projection on the observed gas velocity fields in barred galaxies

The problem of determining the pattern of gas motions in the central regions of disk spiral galaxies is considered. Two fundamentally different cases—noncircular motions in the triaxial bar potential and motions in circular orbits but with orientation parameters different from those of the main disk—are shown to have similar observational manifestations in the line-of-sight velocity field of the gas. A reliable criterion is needed for the observational data to be properly interpreted. To find such a criterion, we analyze two-dimensional nonlinear hydrodynamic models of gas motions in barred disk galaxies. The gas line-of-sight velocity and surface brightness distributions in the plane of the sky are constructed for various inclinations of the galactic plane to the line of sight and bar orientation angles. We show that using models of circular motions for inclinations i>60° to analyze the velocity field can lead to the erroneous conclusions of a “tilted (polar) disk” at the galaxy center. However, it is possible to distinguish bars from tilted disks by comparing the mutual orientations of the photometric and dynamical axes. As an example, we consider the velocity field of the ionized gas in the galaxy NGC 972.

A Subarcsecond Resolution Near‐Infrared Study of Seyfert and “Normal” Galaxies. II. Morphology

We present a detailed study of the bar fraction in the CfA sample of Seyfert galaxies, and in a carefully selected control sample of non-active galaxies, to investigate the relation between the presence of bars and of nuclear activity. To avoid the problems related to bar classification in the RC3, e.g., subjectivity, low resolution and contamination by dust, we have developed an objective bar classification method, which we conservatively apply to our new sub-arcsecond resolution near-infrared imaging data set (Peletier et al. 1999). We are able to use stringent criteria based on radial profiles of ellipticity and major axis position angle to determine the presence of a bar and its axial ratio. Concentrating on non-interacting galaxies in our sample for which morphological information can be obtained, we find that Seyfert hosts are barred more often (79% +/- 7.5%) than the non-active galaxies in our control sample (59% +/- 9%), a result which is at the 2.5 sigma significance level. The fraction of non-axisymmetric hosts becomes even larger when interacting galaxies are taken into account. We discuss the implications of this result for the fueling of central activity by large-scale bars. This paper improves on previous work by means of imaging at higher spatial resolution and by the use of a set of stringent criteria for bar presence, and confirms that the use of NIR is superior to optical imaging for detection of bars in disk galaxies.

Orbits supporting bars within bars

High-resolution observations of the inner regions of barred disc galaxies have revealed many asymmetrical, small-scale central features, some of which are best described as secondary bars. Because orbital time-scales in the galaxy centre are short, secondary bars are likely to be dynamically decoupled from the main kiloparsec-scale bars. Here we show that regular orbits exist in such doubly barred potentials, and that they can support the bars in their motion. We find orbits in which particles remain on loops: closed curves which return to their original positions after two bars have come back to the same relative orientation. Stars trapped around stable loops could form the building blocks for a long-lived, doubly barred galaxy. Using the loop representation, we can find which orbits support the bars in their motion, and the constraints on the sizes and shapes of self-consistent double bars. In particular, it appears that a long-lived secondary bar may exist only when an inner Lindblad resonance is present in the primary bar, and that it would not extend beyond this resonance.

Stellar bars in disk galaxies: from banyans to butterflies?

We provide a non-linear orbit analysis of an evolving self-consistent model galaxy consisting of a two-component disk (stars+gas) and a live halo. We find that dynamical instabilities become milder in the presence of the gas component, and that the stability of planar and 3D stellar orbits is strongly affected by the related changes in the gravitational potential – both are destabilized with the gas accumulation at the center. This results in the overall weakening of the stellar bar and its dissolution on the time scale of a few 10 9 years.

Barred galaxy resonance rings: analytically explaining morphology and predicting dissipative misalignment

Many barred disc galaxies show rings of gas clouds and young stars thought to be in periodic orbits near the two-fold inner and outer Lindblad resonances (ILR and OLR) plus a four-fold ultraharmonic resonance (UHR) of the turning bar with oscillations about the disc orbital motion. To confirm and extend simulations by Schwarz and by Byrd et al. of resonance ring formation, we present an analytical formulation of the clouds' orbital motion which includes dissipative damping of oscillations relative to the local interstellar medium plus the rotation curve, bar pattern speed, and strength. Observed ring morphology matches our plots of periodic orbits where the density is enhanced but clouds do not collide violently. Pairs of ‘outer rings’ bracket the OLR. Dimpled outer rings like that of ESO 507-16 can be matched by plots with strong bars. Slightly dimpled outer rings like that of ESO 509-98 can be matched by weak bar plots. For flat rotation curves, a pair of two-fold rings bracket the ILR; the smaller can be identified with the tiny ‘nuclear rings’. We find narrow UHR rings just outside this pair as well as just inside the OLR pair. We confirm the identification of the larger ILR ring and the inner UHR ring with ‘inner rings’. Disagreeing with the common identification, we associate the dimpled outer rings with the UHR just inside the OLR. See ESO 507-16 as an example. We predict that damping can misalign the ILR and OLR rings relative to the bar as seen in our match to ESO 507-16. We find that for weak bars, if the linearly rising portion of the rotation curve is a significant fraction of the corotation radius, nuclear and inner rings are absent with outer rings still present. We show this in a match to ESO 509-98. Success of the matches to ESO 507-16 and 509-98 shows how the analytic formulation can be used to estimate disc orientation and pattern speed if rotation curve observations are available.

Bar-Driven Gas Structure and Star Formation in the Center of M100

view Abstract Citations (60) References (85) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Bar-Driven Gas Structure and Star Formation in the Center of M100 Sakamoto, Kazushi ; Okumura, Sachiko ; Minezaki, Takeo ; Kobayashi, Yukiyasu ; Wada, Keiichi Abstract High-resolution (2.6" x 2.4") CO(J=1-0) interferometry and near-infrared (J,H,K') imaging were made toward the Virgo spiral galaxy M100 (NGC 4321) to study gas kinematics and its relation to active phenomena in the center of a weakly barred disk galaxy. In the central 1 arcmin (5.0 kpc: D = 17.1 Mpc) of M100, most of the molecular gas distributes within a ~3 kpc diameter region. In this region, there are prominent two-armed molecular spiral arms. The arms start at the ends of the nuclear stellar bar of ~16" (1.3 kpc) long seen in the near-infrared images. There is also a concentration of molecular gas with a diameter of ~3" (250 pc) at the dynamical center of the galaxy. Noncircular motion of the molecular gas is clearly seen and the deviation from circular motion is largest at the arms. From comparisons to theoretical models, we propose that the gas arms are formed through the orbit crowding of molecular clouds at the outer inner Lindblad resonance (OILR: r ~ 1 kpc), and the nuclear gas concentration is formed through infall of self-gravitating gas from the inner inner Lindblad resonance (IILR: r <~ 0.7 kpc). The gaseous structure is driven by the nonaxisymmetric gravitational potential, which is mainly caused by the nuclear stellar bar in the central region. It is shown that analytical and numerical models based on the idea successfully reproduce the gaseous morphology and kinematics observed in the central region of Ml00. Massive star formation in the center of M100 is confined to the central 3 kpc gas distribution, and hotspots seen in Hα are along the molecular spiral arms. Since the molecular spiral arms are the sites of orbit crowding and gas accumulation, cloud collisions, shocks, and gravitational instability are expected. The hotspots are probably attributed to these dynamical features at the OILR. The nuclear concentration of molecular gas induced also by the bar is a clear example of the bar-induced gas fueling to the nuclei of galaxies often suggested as a mechanism for feeding AGNs, though M100 does not show conspicuous AGN activity. Publication: The Astronomical Journal Pub Date: November 1995 DOI: 10.1086/117670 Bibcode: 1995AJ....110.2075S Keywords: GALAXIES: INDIVIDUAL: NGC 4321; GALAXIES: SPIRAL full text sources ADS | data products SIMBAD (4) NED (1)

A dynamical instability of bars in disk galaxies

STRONG, rapidly rotating, persistent bars readily form in numerical simulations of initially axisymmetric disk galaxies1–4. The global dynamical instability responsible for this behaviour is a great embarrassment to the subject of galactic dynamics, as most galaxies in the sky do not possess a strong bar5; the suggestion3 that the dark matter content of galaxies might resolve this discrepancy is unattractive6. Here we present the results of three-dimensional simulations which show that the bar is dynamically unstable to buckling out of the galactic plane (the majority of previous disk galaxy simulations have been strictly two-dimensional). Stars acquire large motions normal to the plane, giving the bar a peanut shape through a mechanism that we suggest to be the fire-hose instability7–9. Bars may be weakened or even destroyed by this instability; thus the fraction of disk galaxies containing strong bars today could be lower than the fraction in which they have formed in the past. The instability may also account for the peanut morphology of many galaxies, and because it leads to a less flattened stellar system with increased central density, it may play a part in the formation of galactic bulges.